The present invention relates to an apparatus and a method for sensing the presence of a liquid by observing the temperature behavior of a heated temperature sensor. The principle of operation of the present invention is to determine whether the sensor is surrounded by a gas or a liquid or to determine which of two immiscible liquids surround the sensor by determining the external thermal load upon the sensor. The thermal load upon the sensor is determined by heating the sensor by application of a predetermined amount of thermal energy and observing the rate of temperature increase of the sensor. If the temperature sensor is surrounded by a gas, there is less thermal conduction away from the sensor than if same sensor was surrounded by a liquid. That is, a gas would absorb less of the thermal energy within the temperature sensor via convection than would the liquid. As a consequence, for a given amount of thermal energy applied to the temperature sensor, the sensor would reach a greater temperature in a gas than in a liquid. A similar condition would occur if the temperature sensor could be immersed in one of two immiscible liquids having differing thermal conductivities. Thus, observation of the rate of temperature increase of the temperature sensor enables a determination of the type of fluid surrounding the sensor.
The above mentioned scheme for determining the presence of a liquid has a problem in that the rate of temperature rise is dependent not only upon the type of fluid surrounding the temperature sensor, but also upon the initial temperature of both the sensor and the fluid. Therefore, in order to employ this method of liquid level sensing, it is necessary to compare the temperature of the temperature sensor with a reference signal which has an initial value dependent upon the initial temperature of the sensor and a rate of change dependent upon both the initial temperature of the sensor and upon the rate of heating of the sensor.